Conventionally, an example of the swing internal contact type planetary gear device is disclosed in a Japanese Patent Application published as JP2002-266955 A. This swing internal contact type planetary gear device is of a cycloid reduction gear device type having an eccentric body rotating about an input/output axial line and having an eccentric axial line deviated from the input/output axial line as a center, an internally toothed gear wheel formed with a plurality of internal teeth provided about the center of the input/output axial line and an externally toothed gear wheel supported on the eccentric body for relative rotation therewith and formed with a plurality of external teeth provided about the center of the eccentric axial line, the externally toothed gear wheel being swingably relatively rotating with the internally toothed gear wheel in engagement therewith. In such cycloid reduction gear mechanism, each internal tooth of the internally toothed gear wheel serves as a pin and each external tooth of the externally toothed gear wheel is formed to have a trochoid tooth profile.
By thus configuring the reduction gear device, the number of tooth differences between the internally toothed gear wheel and the externally toothed gear wheel can be minimized and accordingly, a high reduction gear ratio can be obtained. Further, according to this structure, since a high meshing ratio between the internally toothed gear wheel and the externally toothed gear wheel can be obtained, these gear wheels are known to have a gear with high rigidity and tensile. In this structure, further higher reduction ratio may be achieved due to the increase of the number of teeth in each internally toothed gear wheel and externally toothed gear wheel. However, by the pin and trochoid tooth profile meshing engagement structure, the device may have a problem in oversized diameter, if the number of tooth is increased.
While on the other hand, an involute tooth profile is known as a gear wheel, in which a plenty of teeth can be formed on the gear. The gear wheels with involute tooth profile adapted to the swing internal contact type planetary gear device are already disclosed in JP2009-8143 A, JP5 (1993)-34537 B, JP2007-24072 A, JP1 (1989)-210642 A and JP10 (1998)-227340 A, and have become a public knowledge.